Cetane number (or CN) is a measurement of the combustion quality of diesel fuel during compression ignition. It is a significant expression of diesel fuel quality among a number of other measurements that determine overall diesel fuel quality.
Cetane number is actually a measure of a fuel's ignition delay; the time period between the start of injection and start of combustion (ignition) of the fuel. In a particular diesel engine, higher cetane fuels will have shorter ignition delay periods than lower cetane fuels. In other words, higher cetane fuels can be more easily ignited, also implying that ignition may take place at comparatively lower pressure and temperature, which has a direct impact on the crankshaft angular position at combustion.
The fuel quality may greatly vary around the world. As it is well known, the quality of diesel fuel may even differ at regional level, as e.g. within the USA. Unfortunately the variation of fuel quality also has an effect on driving comfort and engine emissions, and it is thus desirable to take this into account.
JP 2006016994 proposes a method of detecting fuel quality, where the Cetane Number is determined based on a pressure profile in the combustion chamber. Such method requires a dedicated pressure sensor, and thus implies modification of the engine design and additional costs.
DE102008010107 discloses a method of fuel classification, which involves determining a complex ignition frequency component of revolution speed spectrum of the engine. A phase corresponding to this complex component is used as indicator to assign the fuel to one of the different fuel classes.
The phase information corresponding to a complex component of the engine rotation frequency domain has also been used for misfire detection.